1. Field of the Invention
The present invention relates to glass ceramics, and more particularly to an improvement of CaO--Al.sub.2 O.sub.3 --P.sub.2 O.sub.5 system glass ceramics which are used as medical materials, which contain calcium phosphate crystals and are suitable as dental material, artifical bone, etc.
2. Prior Art
In recent years, ceramics are expanding their field of application tremendously, and are now even being used in the medical field. Also being attempted is the application of ceramics to medical materials, such as artificial dental material, for which metals and plastics have been conventionally used.
The inventors of this invention have conducted studies in order to find a ceramic material having an affinity for the living body and to establish a method to manufacture high strength ceramics in any preferred shape with high precision. As a result, it was found that the foregoing objects can be accomplished by using a calcium phosphate system material as the raw material.
Such calcium phosphate system material has several outstanding characteristics, but also has the following disadvantages such as when it is vitrified and crystallized internal defects are caused by the differences in specific gravity between glass and crystals, resulting in lowering the mechanical strength of the ceramic material. This, in turn, causes another problem in that bacteria can then enter the crystal through these defects.
For example, the specific gravity of calcium phosphate glass, which has calcium to phosphorus atomic ratio Ca/P of 0.48, is 2.63. When it is crystallized by heat treatment, and CaO.multidot.P.sub.2 O.sub.5 is produced. The true specific gravity of this CaO.multidot.P.sub.2 O.sub.5 crystal is 2.85, which is considerably higher than the specific gravity of the glass; however, because crystallization proceeds with a surface crystallization mechanism, almost no volumetric decrease occurs after crystallization occurs. An increase in specific gravity by crystallization means the volume to be decreased, and if the total volume is not changed, voids corresponding to the increase in specific gravity are formed inside of the crystal. These voids tend to bring about internal defects, such as cracks and pores. When these internal defects occur, the strength of the glass ceramics is lowered in comparison with that having no defects. Furthermore, since the size of pores formed and their distribution varies depending on various factors such as the atomic ratio of calcium to phosphorus of the glass, the crystallization temperature, the duration of crystallization, etc., it is inevitable that the mechanical strength of the glass ceramics also varies widely.
Likewise, these internal defects also occur in glass ceramics crystallized by a bulk crystallization mechanism although the extent of internal defects is somewhat lower, since some extent of volumetric shrinkage in bulk crystallization occurs.
The abovementioned problems can be solved if the difference in specific gravity before and after crystallization is eliminated. The inventors of this invention have sought in various ways for a method to prevent internal defects while maintaining the characteristic features of calcium phosphate that is an excellent affinity for the living body and a good cavitability, intact. The inventors succeeded in solving the problems mentioned above by employing a CaO--Al.sub.2 O.sub.3 --P.sub.2 O.sub.5 three component glass ceramics (Japanese Patent Provisional Publication No. 1986-186247).
That is, the crystals, which are formed when the glass of the CaO--Al.sub.2 O.sub.3 --P.sub.2 O.sub.5 three component system are crystallized, are predominantly CaO.multidot.P.sub.2 O.sub.5, 2CaO.multidot.P.sub.2 O.sub.5, and Al.sub.2 O.sub.3 .multidot.P.sub.2 O.sub.5. The true specific gravity of each of these components is 2.85, 3.09, 2.59, respectively, and the specific gravity of Al.sub.2 O.sub.3 .multidot.P.sub.2 O.sub.5 is comparatively lower than the other two types of calciuum phosphate crystals. For example, the specific gravity of a glass composed of 21.8 weight % of CaO, 9.3 weight % of Al.sub.2 O.sub.3, and 68.9 weight % of P.sub.2 O.sub.5 is 2.64. When this glass is crystallized, if the crystals formed consist of the abovementioned two types of calcium phosphates alone, the specific gravity of those crystals is higher than that of the glass. As a result, volumetric shrinkage is caused resulting in distortion of the external form, or, if volumetric shrinkage is not caused, defects, such as pores, occur inside the crystals. However, this glass also contains Al.sub.2 O.sub.3, therefore, Al.sub.2 O.sub.3 .multidot.P.sub.2 O.sub.5 that is low in specific gravity is formed concurrently. If crystals of Al.sub.2 O.sub.3 .multidot.P.sub.2 O.sub.5 are produced in a sufficient amount, their volumetric expansion makes up for the volumetric shrinkage caused by the crystallization of calcium phosphate. As a result, the specific gravity after crystallization becomes 2.65. Thus, there is almost no difference in the specific gravity, and internal defects are not formed.
However, as the amount of Al.sub.2 O.sub.3 is increased in order to decrease the difference in specific gravity before and after crystallization, the viscosity of the molten fluid increases. As a result, particularly in casting a tooth crown having a complicated shape, the casting of the molten fluid to the edges of the mold becomes impossible, making it actually infeasible to perform casting.
Moreover, when the amount of Al.sub.2 O.sub.3 is further increased, both the softening point of the glass and the crystallization temperature of calcium phosphate increase. Due to the above, cracks may be caused by thermal stress during crystallization. Or, if crystallization is carried out at a low crystallization temperature to prevent the formation of cracks, the crystals Al.sub.2 O.sub.3 .multidot.P.sub.2 O.sub.5 which are supposed to offset the difference in specific gravity are not sufficiently formed. This in turn causes the problem of making it impossible to prevent the formation of voids and cracks due to the difference in specific gravity before and after crystallization.